The possibility of a tsunami

Page city
Glen Canyon Dam, a concrete arch dam on the Colorado River in Arizona, just north of Page. Its reservoir is called Lake Powell, and is the second largest artificial lake in the USA.
The dam has been a controversy from the start. The dam is anchored in unstable Navajo sandstone (referred to as "solidified sand dunes") and nearly failed in 1983 as the result of an upper Colorado River flood. Siltation, degradation of concrete reinforcements, spillway operational problems and unstable dam abutments all affect the dam's safety. It is estimated that breaching the dam will produce a cool mega-tsunami that would overtop the Hoover Dam by 70 m (230 feet). First operation in 1964, combined with an estimated lifespan between 85 and 100 years, means there's one heck of a party comin' up.... 

 Of course other estimates, suggest a lifespan between 500 and 700 years. There's always people willing to spoil a good party!